Many applications for video/audio production and transmission within a broadcast studio require that equipment within the broadcast studio be synchronized to a common reference. Currently, many broadcast studios conform to an NTSC standard, including synchronization of studio equipment to an NTSC reference, which equipment receives and synchronizes to NTSC timing signals from an NTSC distribution network of the studio. Many broadcast studios, however, will soon broadcast high definition television (HDTV) signals.
An HDTV digital broadcast studio may be required to produce video/audio data streams compliant with specifications set forth in the Advanced Television Standards Committee (ATSC) Digital Television Standard. These ATSC-compliant data streams are required to be compliant with the MPEG-2 systems level standards as set forth in ISO/IEC 13818-1 recommendation H.222.0. Equipment of a broadcast studio may dynamically switch between program sources to produce a compliant output video data stream for transmission. File servers, tape players, encoders, satellite links, networks and possibly other program sources may contain either pre-recorded or live video data streams that must be switched at various points throughout the studio. The MPEG-2 standard requires the output video data stream to contain a Program Clock Reference (PCR) that has a frequency tolerance of 30 parts-per-million (ppm) and a rate-of-change specification less than 75.times.10.sup.-3 Hz/sec. The PCR specification permits the PCR to change one cycle in frequency over 131/3 seconds. The MPEG-2 standard further notes that "Sources of coded data should follow a tighter tolerance in order to facilitate compliant operation of consumer recorders and playback equipment."
A conventional NTSC broadcast studio is synchronized (frequency-locked) to a primary NTSC reference that facilitates timing the studio program sources and sinks to a common reference. HDTV digital broadcast studios require distribution of HDTV synchronization signals to HDTV digital devices that are required to process digital streams for broadcast. An NTSC broadcast studio already has a large investment in a synchronization distribution system that synchronizes the various NTSC video components to the NTSC reference. The digital devices, however, require synchronization to a 27-MHz reference clock, not to the horizontal and vertical signals traditionally distributed throughout an NTSC studio by the NTSC distribution network.